In recent years, the field of diagnostic ultrasound has seen the emergence of a so called "real time" ultrasonic B scanning examination system. The term "real time" means that the systems produce successive images at a rapid enough rate so that images are generated faster than the retention rate of the human eye so that moving objects appear in continuous motion. Thus, in real time operation, the course of the study can be influenced by the operator contemporaneously with the actual study, based on his observation of the rapidly produced image succession. This real time feature is considered an improvement over previous forms of ultrasonic examination, in which only a single image is developed slowly and gradually during the course of a study by moving a single transducer about the patient's skin. In addition to allowing the operator to influence the course of the study, real time systems allow for "stop action" images of moving body parts, and also for motion studies.
Real time ultrasonic examination systems have mainly fallen into two general types, i.e., linear scanning and sector scanning. Electronic linear scanning systems utilize a transducer assembly including a large linear array of individual piezoelectric ultrasonic transducer elements. Imaging circuitry fires a succession of different groups of elements in accordance with a predetermined repeated sequence. This produces a succession of resultant ultrasonic beams propagated along respective parallel paths extending outwardly from the transducer assembly. The assembly is held stationary against the patient's body during image generation.
This technique, in conjunction with known forms of imaging circuitry and display apparatus, produces from received ultrasonic echoes information defining a two dimensional rectangular image of the internal body structure of the patient taken in a common plane, or "slice" through part of the body near the transducer array. One coordinate of each point on the image plane is determined by the amount of time required for incident ultrasonic energy to be reflected back to the transducers from a tissue interface within the body. The other coordinate is determined by the location, along the transducer array, of the axis of the resultant ultrasonic beam which caused the reflected energy.
By operating this system to repeatedly step the incident beam origin along the linear transducer array at, for example, thirty repetitions per second, the rapid sequence of ultrasonically produced image frames which result can show motion of a moving body part. Alternately, a single frame of image data can be held for display, in order to stop rapid motion of such a body part.
The display area scanned by such linear step scanners is rectangular and suitable for presentation on a two dimensional display system, such as a CRT. The electronics required for such a system are relatively inexpensive and simple, since all the beams are parallel and stepped over uniform increments. Moreover, linear stepped scanning systems exhibit substantially uniform field of view throughout their display area.
Linear systems, however, do have some disadvantages. For example, the transducer assembly must of necessity be rather long, and therefore clumsy to use, since the length of one side of the rectangular display equals the length of the transducer array. Since all the ultrasonic beams produced by the linear scanner are propagated along parallel lines, the linear scanner is not generally capable of imaging portions of the patient's body which are hidden behind other nearer portions, such as an organ which may be located behind a rib.
A known type of electrically stepped linear array ultrasonic system is described in the following publication, which is hereby expressly incorporated by reference: Havlice, J. F., et al, "Medical Ultrasonic Imaging: An Overview of Principles and Instrumentation," Proceedings of the IEEE, Vol. 67, No. 4, April, 1979, pp. 620-641.
Another type of known real time electronic ultrasonic scanner is the electronic sector scanner. In such devices, a linear array of transducer elements is employed as in the case of linear step scanning. The length of the array, however, is considerably shorter than in the case of the step scanned linear device described above.
In using the electronic sector scanner, the transducer assembly is held stationary near the portion of the patient's body to be examined. All elements are repeatedly fired in a single group. Phase delay circuitry is associated with imaging circuitry which is utilized to control ultrasonic beam emission and reception by the transducer elements. By proper phase delay of respective elements, the ultrasonic beam repeatedly produced by the transducer array is "steered" at different angles to the face of the transducer assembly. The angle of the ultrasonic beams produced by successive firings of all the elements of the transducer array is repeatedly scanned in increments from one side to another, such that the successive ultrasonic beams collectively sweep through the patient's body at different angles in a common plane.
Several advantages over the linear stepped scanner are achieved by use of the electronic sector scanner. First, the transducer assembly is significantly more compact than in the case of the stepped scanner, and can thus be used at almost any location on the patient's body. Since the ultrasonic beams are directed into the subject at different angles, the electronic sector scanner can image portions of the body which might be hidden from view of the linear stepped scanner because of their location behind other more opaque portions of the body, such as bone.
Electronic sector scanning, however, does have its own inherent disadvantages. One such disadvantage is that these scanners have a narrow field of view in regions of the body close to the transducer assembly. This is because the field of view of the sector scanner resembles a sector of a circle and, close to the transducer assembly, the excursion of the sweep of the ultrasonic beam is quite small.
Another disadvantage of the electronic sector scanner is the relatively high cost of such units, due in large measure to the complexity of the electronics necessary to achieve the delay scheme employed to effect beam steering. While a typical linear transducer step scanner costs in the neighborhood of $15,000 to $30,000, the corresponding range of cost for electronic sector scanners is about $65,000 to $100,000 each.
Mechanically steered real time linear and sector scanners, using oscillating or rotating single crystal transducers, have also been proposed. Such systems, however, suffer from relatively large physical size, and problems associated with reliability of the mechanical drive. They also usually require the transducer to be immersed in a fluid.
Known proposals for electronic and mechanical sector scanners are described in the above referenced Havlice, et al., publication.
Another system, a variant of ultrasonic step scanning, (Bushmann "New Equipment and Transducers for Ophthalmic Diagnosis," Ultrasonics, Vol. 3, pages 18 et seq, January-March, 1965,) has been proposed relating to ultrasonic examination of the eye. It is suggested to ultilize ten transducers arranged in an arc such that the ultrasonic beams emitted by each of the transducers mutually converge near the center of the eye ball. Pulsing circuitry is applied to separately fire each of the transducers in a sequence.
A disadvantage of this type of examination stems from the fact that tissue interface points within the patient's body which generate ultrasonic echoes may be struck by primary incident energy from more than one transducer. Each such point could thereby lack uniqueness of location on the image display, causing blurring.
This lack of unique location of multiply-struck interface points would be caused by inhomogeneity in the patient's body. Acoustic velocity differs among tissue types. If the time required for an ultrasonic echo from one transducer to return to that transducer from the subject point is different from the corresponding return time with respect to another transducer whose energy also strikes the point, the subject point will show up at slightly different spots on the display.
It is an object of this invention to provide an ultrasonic scanning system having the flexibility, compactness and swept beam characteristics of an electronic sector scanner without the sector scanner's limited close up field of view and high price, while preserving the uniqueness of display location for each imaged point, all for roughly the cost of a simple linear step scanner.